[ruby-core:111920] [Ruby master Feature#18285] NoMethodError#message uses a lot of CPU/is really expensive to call

20 Jan 2023

      Issue #18285 has been updated by mame (Yusuke Endoh).

Discussed at the dev meeting. @matz is positive about the change and said he would give it a try. The message format is slightly changed as follows:

* If the receiver is a Class, `... for class <class name>` should be printed.
* If the receiver is a Module, `... for module <module name>` should be printed.
* If the receiver is an extended object (i.e., has a singleton class), `any_to_s` should be used.
* Otherwise, `... for an instance of <class name>` should be printed.

Examples:

```
42.time    #=> undefined method `time' for an instance of Integer (NoMethodError)

class Foo
  privatee #=> undefined local variable or method 'privatee' for class Foo (NoMethodError)
end

def (o=Object.new).foo
end
o.bar      #=> undefined method `bar' for #<Object: 0xdeadbeef(any_to_s)> (NoMethodError)
```

I'll update my PR later. When we use the new message format and find it inconvenient, we will adjust it accordingly.

----------------------------------------
Feature #18285: NoMethodError#message uses a lot of CPU/is really expensive to call
https://bugs.ruby-lang.org/issues/18285#change-101343

* Author: ivoanjo (Ivo Anjo)
* Status: Open
* Priority: Normal
----------------------------------------
Hello there! I'm working at Datadog on the ddtrace gem -- https://github.com/DataDog/dd-trace-rb and we ran into this issue on one of our internal testing applications. I also blogged about this issue in <https://ivoanjo.me/blog/2021/11/01/nomethoderror-ruby-cost/>.

### Background

While testing an application that threw a lot of `NoMethodError`s in a Rails controller (this was used for validation), we discovered that service performance was very much impacted when we were logging these exceptions. While investigating with a profiler, the performance impact was caused by calls to `NoMethodError#message`, because this Rails controller had a quite complex `#inspect` method, that was getting called every time we tried to get the `#message` from the exception.

### How to reproduce

```ruby
require 'bundler/inline'

gemfile do
  source 'https://rubygems.org'

  gem 'benchmark-ips'
end

puts RUBY_DESCRIPTION

class GemInformation
  # ...

  def get_no_method_error
    method_does_not_exist
  rescue => e
    e
  end

  def get_runtime_error
    raise 'Another Error'
  rescue => e
    e
  end

  def inspect # <-- expensive method gets called when calling NoMethodError#message
    Gem::Specification._all.inspect
  end
end

NO_METHOD_ERROR_INSTANCE = GemInformation.new.get_no_method_error
RUNTIME_ERROR_INSTANCE = GemInformation.new.get_runtime_error

Benchmark.ips do |x|
  x.config(:time => 5, :warmup => 2)

  x.report("no method error message cost") { NO_METHOD_ERROR_INSTANCE.message }
  x.report("runtime error message cost") { RUNTIME_ERROR_INSTANCE.message }

  x.compare!
end
```

### Expectation and result

Getting the `#message` from a `NoMethodError` should be no costly than getting it from any other exception.

In reality:

```
ruby 3.0.2p107 (2021-07-07 revision 0db68f0233) [x86_64-linux]

no method error message cost
                        115.390  (± 1.7%) i/s -    580.000  in   5.027822s
runtime error message cost
                          6.938M (± 0.5%) i/s -     35.334M in   5.092617s

Comparison:
runtime error message cost:  6938381.6 i/s
no method error message cost:      115.4 i/s - 60130.02x  (± 0.00) slower
```

### Suggested solutions

1. Do not call `#inspect` on the object on which the method was not found (see <https://github.com/ruby/ruby/blob/e0915ba67964d843832148aeca29a1f8244ca7b1/error.c#L1962>)
2. Cache result of calling `#message` after the first call. Ideally this should be done together with suggestion 1.

-- 
https://bugs.ruby-lang.org/